1 Field of the Invention
The present invention relates to an exhaust gas purifying catalyst and a method of producing the exhaust gas purifying catalyst.
2 Description of the Related Art
There have been known exhaust gas purifying catalysts which comprise two catalytic layers, a first one of the two catalytic layers containing at least one noble metal selected from a group of Pt, Rh and Pd which is partly or entirely supported on ceria, Ba and K and a second one of the two catalytic layers containing at least one noble metal selected from the group of Pt, Rh and Pd but not containing K, which are formed in the form of double layers on a substrate. Such an exhaust gas purifying catalyst shows an increased ability to absorb NOx in a lean atmosphere (which refers to the atmosphere containing oxygen in excess as compared with exhaust gas produced with combustion of a stoichiometric air-to-fuel mixture) by virtue of the first catalytic layer and functions as a three-way catalyst in exhaust gas produced from combustion of a stoichiometric air-to-fuel mixture. In the exhaust gas purifying catalyst K and Na work as NOx absorbing component and the noble metal supported on ceria works as a promoter for NOx absorption. One of such exhaust gas purifying catalyst is known from, for example, Japanese Unexamined Patent publication No. 8-281106. Another type of exhaust gas purifying catalyst that is known from, for example, Japanese Unexamined Patent publication No. 9-85093, comprises an alumina coating layer impregnated with a noble metal such as Pt formed on a substrate, the alumina coating layer being further impregnated with K, Na and Li. In the exhaust gas purifying catalyst these K, Na and Li are made apt to produce a compound carbonate which puts the exhaust gas purifying catalyst in an unstable state so as to show improved NOx adsorbing and desorbing characteristics and makes it recover from sulfur-poisoning to gain high NOx conversion efficiency. Still another type of exhaust gas purifying catalyst such as known from, for example, Japanese Unexamined Patent publication No. 10-151357 comprises an alumina under layer formed on a honeycomb substrate and an over layer formed on the under layer. These under and over layers are impregnated with a mixture of a solution of a NOx absorbent such as Ba, K and Na and a solution of a noble metal with an effect of improving the dispersibility of the NOx and the noble metal.
It is an object of the invention to provide an exhaust gas purifying catalyst which has improved resistance to sulfur-poisoning ability of a NOx absorbent such as Ba adapted to absorb NOx in the lean atmosphere (e.g. the exhaust gas with oxygen concentration higher than 4% for example from combustion of a lean air-fuel mixture at an air-to-fuel ratio ranging from 16 to 100) so as thereby to increase its NOx conversion efficiency.
It is another object of the present invention to provide a method of producing an exhaust gas purifying catalyst which has improved resistance to sulfur-poisoning of a NOx absorbent such as Ba adapted to absorb NOx in the lean atmosphere, e.g. in the exhaust gas from combustion of a lean air-to-fuel mixture with an air-to-fuel ratio ranging from 16 to 100 so as thereby to increase its NOx conversion efficiency.
In order to accomplish the foregoing objects of the present invention, an exhaust gas purifying catalyst contains a combination of a NOx absorbing component such as Ba and a K component or a Na component. The NOx absorbing component comprises at least one kind of element selected from a group of alkaline earth metals, a group of alkaline metals and a group of rare earth elements. In the exhaust gas purifying catalyst the K component or the Na component is distributed so as to be made exposed to an exhaust gas prior to the NOx absorbing component. While it has been known in the art that K and Na have the function of absorbing NOx, noticing the respect that K and Na are able to absorb S, the exhaust gas purifying catalyst is adapted so that the NOx absorbing component is less poisoned with S by causing the K component or the Na component to contact with an exhaust gas prior to contact of the NOx component with the exhaust gas so as thereby to absorb S contains more than 7 ppm in the exhaust gas. Although the K component or the Na component is transformed into the form of a carbonate as a result of calcination during preparing the exhaust gas purifying catalyst, when encountering a sulfuric component in an exhaust gas, it absorbs the sulfuric component and is consequently transformed into a sulfate such as K2SO4or Na2SO4. However, when the exhaust gas lowers its oxygen content, e.g. when a stoichiometric air-to-fuel ratio is reached, the sulfate is easily decomposed into carbonate. In particular, in the case of a combination of NOx absorbing component and K, even after the catalyst is exposed to a high temperature exhaust gas, it shows a high NOx conversion efficiency and is therefore favorable to improving its heat resistance. A component selected from a group of alkaline earth metals, a group of alkaline metals and a group of rare earth elements, in particular Ba, is preferable as the NOx absorbing component. Moreover, a combination of Ba and another NOx absorbing component may be employed.
In the case of placing a K component or a Na component in an exhaust gas stream upstream from the NOx absorbing component so as to expose the K component or the Na component to the exhaust gas prior to the NOx absorbing component, the catalyst may be comprised of a catalytic layer containing a K component or a Na component (which is referred to as a second catalytic layer) and a catalytic layer containing NOx absorbing component (which is referred to as a first catalytic layer) which are coated side by side on a substrate and placed in an exhaust gas stream so that the second catalytic layer is positioned upstream from the first catalytic layer. Otherwise the catalyst may be preferably comprised of the first catalytic layer formed as an under catalytic layer on a substrate and the second catalytic layer formed as an over catalytic layer over the first catalytic layer. In such the catalyst the first catalytic layer is preferable to contain a noble metal, such as, Pt or Rh in addition to the NOx absorbing component. The noble metal, on one hand, converts NOx in an exhaust gas while the exhaust gas is at higher oxygen contents and, on the other hand, converts NOx that is discharged from the NOx absorbing component while the exhaust gas is at lower oxygen content. As support of the noble metal alumina, preferably xcex3-alumina and/or cerium may be employed. Since CeO2has oxygen absorption ability (O2storage effect), when the engine operates at an approximately stoichiometric air-to-fuel ratio, i.e. an excess air ratio (of approximately one (1), it causes the catalyst to act as a three-way catalyst, so as to effectively lower levels of other emissions, such as, hydrocarbon (HC) and carbon monoxide (CO) in addition to NOx.
The second catalytic layer is preferable to contain zeolite supporting a transition metal component in addition to a K component or a Na component. The transition metal supported on the zeolite activates NO and HC, so as to increase the NOx conversion efficiency of the catalyst. That is, NO in an exhaust gas is oxidized to NO2which is easily absorbed by the NOx absorbing component, and HC is activated through partial oxidization and cracking. The activated HC promotes a reduction-decomposition reaction of NOx. Noble metals, particularly Pt is preferable as the transition metal and using Pt together with Rh is more preferable. MFI type is preferable as zeolite.
K component or Na component can be included not only in the second catalytic layer but also in the first catalytic layer. In this case it is preferable that the second catalytic layer has a higher concentration of those components than the first catalytic layer. This is because it can reduce an amount of S to diffuse into the first catalytic layer by making S in the exhaust gas first absorbed into the K component or the Na component in the second catalytic layer over the first catalytic layer, which prevents the NOx absorbent in the first catalytic layer from sulfur-poisoning.
NOx absorbing component may also be included in both first catalytic layer and second catalytic layer. In this case it is preferable that the concentration of K component or Na component in the second catalytic layer is higher than the concentration of the NOx absorbing component in order to cause S in the exhaust gas to be absorbed into the K component or the Na component so as thereby to prevent the NOx absorbing component from sulfur-poisoning.
A method suitable for producing the above-mentioned exhaust gas purifying catalyst comprises the steps of:
forming a first layer containing alumina on a substrate;
forming a second layer containing zeolite over the alumina containing layer;
impregnating both alumina containing layer and second zeolite containing layer with a solution of a mixture of either one of K or Na and at least one element selected from a group of alkaline metals other than K and Na, alkaline earth metals and rare earth elements;
drying the alumina containing layer and the zeolite containing layer impregnated with the solution, and
calcining the alumina containing layer and the zeolite containing layer after drying.
This method is corroborated by the fact that K ions or Na ions become carried more in the zeolite containing layer because K ions or Na ions are adsorbed by the zeolite more easily and move in a solution more easily than ions of alkaline metals other than K and Na, alkaline earth metals and rare earth elements. The movement of the ions in a solution is made while a solvent of the solution evaporates from the alumina containing layer and the zeolite containing layer during calcining these layers after the impregnation.
As explained above, the exhaust gas purifying catalyst of the invention comprising NOx absorbing component and a K component or a Na component in which the K-component or the Na-component is brought into contact with an exhaust gas before the NOx absorbing component is brought into contact with the exhaust gas can controlling sulfur-poisoning of the NOx absorbing component as a result of absorption of S in the exhaust gas by the K component or the Na component and has the advantage of increasing the NOx conversion efficiency thereof. Furthermore a combination of the K-component and the NOx absorbing component is contributory to improvement of heat resistance of the exhaust gas purifying catalyst.
According to the invention of producing the catalyst for purifying exhaust gas mentioned above, the catalyst is produced by the steps comprising: forming a layer containing alumina on a support material; forming a layer containing zeolite on the layer containing alumina; impregnating both alumina containing layer and zeolite containing layer with a solution of a mixture of either one of K or Na and at least one element selected from a group of alkaline metals other than K and Na, alkaline earth metals and rare earth elements, and then drying and calcining the layers. Thus the catalyst which carries a larger amount of K or Na in the outer layer, i.e. the containing zeolite layer, than the under layer can be obtained.